In many ways, the chemical and biological properties of peptides make them very attractive candidates for use as therapeutic agents. Peptides are naturally occurring molecules made up of amino acid building blocks, and are involved in countless physiological processes. With 20 naturally occurring amino acids, and any number of non-naturally occurring amino acids, a nearly endless variety of peptides may be generated. Additionally, peptides display a high degree of selectivity and potency, and may not suffer from potential adverse drug-drug interactions or other negative side effects. Moreover, recent advances in peptide synthesis techniques have made the synthesis of peptides practical and economically viable. Thus peptides hold great promise as a highly diverse, highly potent, and highly selective class of therapeutic molecules with low toxicity.
A number of peptides have been identified as therapeutically promising; however in vitro results have often not proven to bear out in vivo. Significantly, peptides suffer from a short in vivo half life, sometimes mere minutes, making them generally impractical, in their native form, for therapeutic administration. Thus there exists a need in the art for modified therapeutic peptides having an enhanced half-life and/or reduced clearance as well as additional therapeutic advantages as compared to the therapeutic peptides in their unmodified form.